Characterization of the hydrophobicity of mesoporous silicas and clays with silica pillars by water adsorption and DRIFT.

The hydrophobic-hydrophilic properties of a solid are related to the material chemistry and, often, these properties are relevant to the applications of a particular material. Contrarily to what happens with other properties, such as specific surface areas or pore volumes, the methodologies to ascertain on the hydrophilicity of a porous material are not well defined. In this work, we discuss and relate the information on the hydrophobicity degree obtained from water adsorption isotherms and from diffuse reflectance infrared Fourier transform (DRIFT), in a set of porous materials. The studied materials were mainly mesoporous solids, namely of MCM-41 and SBA-15 types, two xerogels and also different porous clays heterostructures. Both techniques were informative on the hydrophobic-hydrophilic properties of the studied samples, but the correlation between the information obtained by each technique was not straightforward. Water adsorption isotherms are much more sensitive to the differences of the studied materials than the DRIFT spectra. For silica-based mesoporous materials with similar surface chemistry, the water adsorption process and hence, the hydrophobic-hydrophilic properties, is mainly dependent on the pore diameters. However, water adsorption is much more sensitive to changes in the nature of the adsorbent surface than to changes in the pore diameter.

A fiber-optic sensor to detect volatile organic compounds based on a porous silica xerogel film.

Fiber-optic sensors are increasingly used for the determination of volatile organic compounds (VOCs) in air matrices. This paper provides experimental results on the sensitivity of a fiber-optic sensor that uses a film of a porous silica xerogel as the sensing element. This film was synthesized by the sol-gel process and affixed to the end of the optical fiber by the dip-coating technique. This intrinsic sensor works in reflection mode, and the transduction takes place in the light that travels through the core of the fiber. The VOCs included in this research cover a wide range of compounds with different functional groups and polarities. The highest sensitivity was for 2-propanol (13.1±1.4 M(-1) nm(-1)), followed by toluene (11.4±1.4 M(-1) nm(-1)), and 1-butylamine (9.5±0.4 M(-1) nm(-1)). Acetone and cyclohexane had the lowest sensitivity of all studied VOCs. Limits of detection varied between 9.1×10(-5) M for 1-butylamine and 1.6×10(-3) M for ethanol. Silanol groups on the xerogel surface act as weak acids and interact strongly with molecules that contain OH groups like alcohols, π-electrons like toluene, or a lone pair of electrons like toluene. Stronger interaction of methanol and ethanol with the silanol groups on the film led to some irreversible adsorption of these analytes at room temperature.

Fabrication and performance evaluation of highly sensitive hybrid sol-gel-derived oxygen sensor films based on a fluorinated precursor.

This paper describes the fabrication and performance of a range of highly sensitive luminescence-based oxygen sensor films based on the fluorinated sol-gel precursor 3,3,3-trifluoropropyltrimethoxysilane (TFP-TMOS). The oxygen-sensitive ruthenium complex [Ruthenium(II)-tris(4,7-diphenyl-1,10-phenanthroline)] dichloride, [Ru(dpp)(3)](2+) was entrapped in a wide range of ORMOSILs (organically modified silicates) matrices composed of alkyl and TFP-TMOS sol-gel precursors in different relative ratios. The influence of TFP-TMOS on sensor sensitivity, humidity-sensitivity and long-term stability was investigated and performance was compared to that of similar but non-fluorinated films. The optimum limit of detection was found to be 0.002% of oxygen for the propyltriethoxysilane (PTEOS)/TFP-TMOS-derived film compared to 0.09% for PTEOS-derived films reported previously. Photobleaching of the luminescent complex in fluorinated and non-fluorinated matrices was also investigated. It was established that photobleaching was reduced but not eliminated in fluorinated films. All films produced in this study exhibit very good reproducibility, reversibility, enhanced sensitivity, humidity-insensitivity and long-term stability.